1. Field of the Invention
This invention relates to thermally responsive valves for use in pollution control systems for automotive internal combustion engines and is particularly suited for controlling the flow of stored fuel vapors from the engine fuel tank to the intake manifold.
2. Discussion of the Prior Art
Evaporator cannisters are presently being used to collect fuel vapors given off by the fuel in the vehicle fuel tank to prevent this form of pollution from entering the atmosphere. The evaporator cannister is one component of a fuel vapor recovery system which in some installations includes a solenoid valve for controlling vapor flow from the cannister to the engine intake manifold. The solenoid valve is opened a predetermined time after engine start-up in order to avoid introducing an excessively rich air-fuel mixture to the engine which causes poor engine performance. Although the solenoid valve type arrangement has been found functionally operative, it has long been desired to find a lower cost alternative to replace the solenoid valve and its associated control circuitry.
As an alternative to the electrically operated solenoid valve other known systems have incorporated a two port vacuum valve actuated by a suitable vacuum signal or a two port valve actuated by a thermally expansible wax-type power element. The performance of these valves upon exposure to prolonged service has been adversely affected by the degrading effects of the fuel vapors on the diaphragms and wax-type power elements.